The invention relates to a method for transmitting encrypted information as recited in the preamble of claim 1. The encryption of data streams should protect against eavesdropping thereof, or other interfering therewith by unauthorized entities in view of the commercial and other hazards that such eavesdropping or interfering could cause. For improved security, the encryption key or keys should be repeatedly exchanged for another key or keys that could not readily be derived from an earlier key. In the present invention, the transmitter, sender, or source entity will initiate the transition to another key, in general, both as regards to the content of the new key(s), and also as regards to the instant of transition. In the ambit of the present invention, the receiver is presumed to know what the next encryption key should be, and therefore, what decryption key to apply next. Now, the receiver entity has somehow to be notified of the exact position in the encyphered data stream where the transition to the new key has taken place, to allow the destination to changeover to an appropriate new decrypting key or keys. The purpose of such notifying is that in principle all encrypted information should be useful to the receiver, and furthermore, that the encryption process should be executed in a straightforward manner, without any necessity for backtracking or other deviation from decrypting with only the proper key or keys.
Now, EP 1 054 546 describes how to include in the encrypted data a binary flag that at certain instants will signal the receiver to changeover to the next decryption key. This procedure will evidently diminish the channel transfer capability for the necessity to repeatedly transmit the flag, and moreover, may also allow an eavesdropper to detect the instant of such changeover.